Mineralogia Rocks

7/28/2019 Mineralogia Rocks

1/19

Mottl, M.J., Davis, E.E., Fisher, A.T., and Slack, J.F. (Eds.), 1994Proceedingsof the Ocean Drilling Program, Scientific Results, Vol. 139

8. MINERALOGY OF HYDROTHERMALLY ALTERED SEDIMENTS AND IGNEOUS ROCKS ATSITES 856- 858, MIDDLE VALLEY, JUAN DE FUCA RIDGE, LEG 1391

Viktor Kurnosov,2 Ivar Murdmaa,3 Tamara Rosanova,3 Georgy Kashintzev,3 Vadim Eroshchev-Shak,2 and Sergey Krasnov4

ABSTRACTMineralogical analyses of hydroth erm ally altered sedim ents and basalts recovered from Leg 139 boreholes in Middle Valleyof the Jua n de Fuca Ridge reveal an increasing intensity of alteration, from the least affected H oles 856A throu gh 856B andcomposite section at Site 857, to the most affected atSite 858, drilled within an act ive hydrothermal vent a rea . Three m ain hydro-thermal zones are recognized in the sed iment sequence at Sites 856, 857, and 858. The lowermost zone , Zone I I I , is stronglyhydrothermally altered and contains pyrite, chlorite, quartz, epidote, wairakite, and analc ime. The m ineral assemblages both instrongly altered sediments and in interbedded basaltic sills ind ica te a l te ra t ion u nder the greensch ist metam orphic fac ies. Zon e IIcontains hydrothermal pyrite, chlorite, a corren site-like minera l , an hydr i te , and gypsum; quar tz and carbonate are also present.T h e uppe r m os t z one , U ppe r Z one I, is weakly altered and contains mainly clastic sediments and authigenic smectites.

INTRODUCTIONLeg 139occupied four sites located in Middle Valley of the N or t h -er n Juan de Fuca Ridge (F ig. 1) to study processes and products asso-

ciated with hydrothermal circulation in a sedimented spreading center.This article reports the results of mineralogical analyses of sedimentsamples from Holes 856A and 856B; Holes 857A, 857C, and 857D;Holes 858A, 858B, 858C, and 858D; and igneous rocks from Holes856A, 857C, 857D, 858F, and 858G. The goal of the article is todiscuss and interpret hydrothermal alteration of the sediments. Thesamples we received from Leg 139contain low or negligible amountsof sulfides; consequently, this article does not elucidate questionsabout sulfide mineralogy.

SITES AND SETTINGSSite 855 was located near the eastern boundary fault scarp- Mafic

igneous rocks were encoun tered at 74 mbsf. Heat flow is low at Site855. Unaltered hemipelagites and turbidites are considered to repre-sent "background" sediment.

Site 856 was located near a small hill in the eastern par t of MiddleValley, about 3 km west of the eastern boundary fault scarp and 4 kmeast of the present- day rift axis. H oles 856A and 856B were locatedat the center and on the southern edge of the hill. There is a smallactive vent field roughly 200 m south of Site 856. Hot (up to 265C)water is discharging there. Coring in Holes 856A and 856B recoveredbasalts from thick sills at 112 mbsf and 120 mbsf, respectively. Thebasalts are overlain by undisturbed semi-indurated P leistocene tur-bidites. Temperatures at the bottom of the holes are about 60C inH o le 856A and 160C in Hole 856B.

Site 857 is located 1.6 km from the hydrothermal vent field at Site858 and 5.2 km west of the eastern boundary fault scarp. The site islocated on a major thermal anomaly, where the seafloor heat flowexceeds 0.8 Wra"2. Hole 857A, located slightly west of the peak inheat flow, was drilled to 112 mbsf in an area with a heat flow of about0.7 Wnr 2. Hole 857C is located 180 m from Hole 857A in an area of

1 Mottl, M.J., Davis, E.E., Fisher, A.T., and Slack, J.F . (Eds . ) , 1994. Proc. ODP, S ci.Results, 139: College Station, TX (Ocean Drilling Program).

2 Geological Institute , Pyzhevsky Per. 7, Moscow 109017, R ussia.3 I n s t i tu te of Oceanology, Krasikov St. 23, Moscow 117218, Russia.4 "Ocean geologia," Moika St. 120, St. Petersburg 190121, Russia.

measured heat flow of about 0.8 Wm 2. This hole was drilled to 567.7mbsf, where a temperature greater t h a n 250C was estimated. Maficsills interlayered with sediments were encountered between 471.1an d 567.7 mbsf. Hole 857D, a deep reentry hole, was drilled 50 mnor t h of Hole 857C to a total depth of 936.2 mbsf. Mafic sills andinterlayered sediments were cored between 581.1 and 936.2 mbsf.

Site 858 is located 1.6km north of Site 857 at an active hydrother-m al vent field where numerous vents with temperatures ranging from255 to 276C have been observed. H ole 858A is located about 100 mwest of the vent field area and about 150 m west of the nearestcurrently active vents. The calculated thermal gradient is 1.7C m"1.H o le 858C is located within the distal part of the vent field area, 70 mwest of the nearest vent. The downhole temperature gradient is 3Cm"1. Hole 858B is located 140 m east of Hole 858C and only a fewmeters away from a 276C hydrothermal vent. A temperature of197C was measured at 19.5 mbsf. Hole 858D is located at the centerof the vent field, about 70 m northeast of the nearest vent (at Hole858B) where a temperature of 208C was measured at 18.5 mbsf.H o le 858F (258 mbsf) and deep reentry Hole 858G (432.6 mbsf) weredrilled approximately in the center of th e field.

METHODSSediments were studied in smear- slides and thin sections. X-ray

diffraction (XRD) mineralogy was determined for bulk powder sam-ples, for clay fraction (less t h a n 0.001 mm), and in rare samples for0.001- 0.01- mm fractions. These fractions were precipitated fromsuspension and prepared as oriented specimens.

Secondary minerals from igneous rocks were studied in thin sec-tions and analyzed by XRD by oriented specimens prepared fromsuspensions using distilled water and by grinding rock samples (1- 2m m ) in an agate mortar. Clay minerals were concentrated in thesuspensions. A few samples were examined by microprobe.

ADRON - 1.5 X- ray diffractometer with C u K emission, Ni- filter,an d slot openings of 0.5, 1, 1, 0.5 mm was used. Each sample wasdried in air, treated with glycerine, and heated at 550C for 1 to 2hours. For quantitative counting of clay minerals the method of Sudoet al. (1969) was used. They proposed the following mean intensitycoefficients: (M on tmor i l lon i t e ) : (H y drom i ca) : (Chlorite) = 3.6 : 1 : 1 .

T h e coarse fraction (0.1-0.05 mm) was separated by wet sievingan d studied using oil immersion. Heavy and light fractions wereseparated in bromoform (2.9 g/cm3). Most samples were examined byscanning electron microscope ( S E M ) .

113


2/19

V. KURNOSOV ET AL.12850' 12840'

4830'

4820' -

Site 856/ Site 855(BH) if

12834'

- 4830'

4820'

12850 12840' 12834'Figure 1. Site mapshowing locations ofSites 855-858 inMiddle Valley.

114


3/19

MINERALS IN SEDIMENTSMinerals

Chlorites, hydromica, sm ectites, and corrensite-like minerals weredetermined in sediments from Leg 139 by routine XRD methods. Fechlorites are widely distributed in the sediments. They h ave low inten-sity (001) reflections at 14 and high-intensity (002) reflections at 7 (Fig. 2, Sample 139-857C-65R-1, 12-14 cm). Hydromica com-monly has a (001) reflection at 9.94 to 9.98 (Fig. 2, Sample 139-857C-27R-2, 81-86 cm), which indicates that hydromica containsblocks with K and Na components. The hydromica is dioctahedral,with a (002) reflection near 4.97 . Sm ectites exhibit the (001) reflec-tion at 17.6-17.8 in glycerine-saturated specimens (Fig. 2, Sample139-857C-11R-1, 103-105 cm). A corrensite-like mineral was foundin sediments from Hole 858B . Typical reflections are shown in Figure3 (Sample 139-858B-2H-4, 68-7 0 cm).The following nonclay minerals were detected by X-ray diffrac-tion, thin section, and immersion analysis: quartz and feldspar, am-phibole, biotite, muscovite, pyrite, iron oxides, anhydrite, gypsum,barite, carbonates, epidote, wairakite, analcime, clinoptilolite (?),rutile, anatase, sphene, zircon, garnet, apatite, zoisite and trace min-erals (chalcedony, opal A, volcanic glass, talc [?], tremolite and/oractinolite, black opaques, olivine, leucoxene).Origin of Minerals

Unaltered hemipelagites and turbidites recovered at Site 855 areconsidered to represent "background." T he Shipboard Scientific Party(Davis, Mottl, Fisher, et al., 1992) determined that "backgro und" sedi-ments consist predominantly of quartz and plagioclase feldspar withsubordinate amounts of am phibole and mica. The clay-size fraction ispredominantly composed of chlorite and hydromica. This mineralassemblage reflects a mainly terrigenous origin.Sediments recovered at Sites 856, 857, and 858 often containaltered or recrystallized detrital minerals and newly-formed quartz,possibly feldspar, amphibole, as well as a group of evidently hydro-thermal minerals (including anhydrite, wairakite, analcime, corren-site-like mineral, often ch lorite).Clay Minerals

Hydrom ica has both a detrital and an authigenic origin. Authigenichydromica as radial aggregates was found by SEM and confirmed bymicroprobe measurement of potassium (Plate 1, Figs. 1-3).Chlorite is also both detrital and authigenic. Often the two typesare intermixed and difficult to distinguish. A uthigenic chlorite is com-mon as cement in siltstone, where it shows an apparently authigenictexture. The claystone is predominantly composed of chlorite, whichis also apparently authigenic, based on intergrowth structures includ-ing spheroidal cabbage-like aggregates and stacks of parallel or di-verging sheets. The crystalline structure of chlorite is well demon-strated by SEM images (Plate 1, Figs. 4-7). Chlorite is authigenic inorigin where the clay fraction is composed of pure chlorite. Smallpores and amygdules are in some cases encrusted or infilled bynewly-formed chlorite. Chlorite also replaces feldspar and mica.The mixed-layer corrensite-like mineral is authigenic (hydrother-mal) in origin. The structure of this mineral is shown in Plate 1, Fig-ure 8.Almost all the smectites are of authigenic origin.Quartz and Feldspar

Quartz and feldspar in the coarse fraction of sed iments are mainlyterrigenous clastic minerals. Thin sections show recrystallization(from weak to full) of the detrital quartz cementing silty sandstoneand clayey siltstone. The mosaic (anhedral) authigenic quartz occursboth in veinlets and as recrystallized sandy laminae. Hydrothermal

MINERALOGY OF SEDIMENTS AND IGNEOUS ROCKS7.0

3.54

4.7

14.0

7.0

B

3.53 14.0

17.6

14.2

3.53

Figure 2. X-ray diffraction patterns of less than 0.001 mm fraction fromsediments. A. Fe chlorite (Sample 139-857C-65R-1, 12-14 cm, air-dried). B.Fe chlorite and hydromica (Sample 139-857C-27R-2,81-86 cm, air-dried). C.Smectite, hydromica, and chlorite (Sample 139-857C-11R-1, 103-105 cm,glycerine-saturated).

115


4/19

V. KURNOSOV ET AL.16 A 32 A

3.56 A 8.0A4.62A

2.92AFigure 3. X-ray diffraction pattern of corrensite-likemineral and chlorite (lessthan 0.001 mm fraction, Sample 139-858B-2H-4, 68-70 cm, glycerine-satu-rated) from sediments.euhedral quartz crystals have grown in fissures and cavity walls.Detrital feldspar is replaced by chlorite, epidote, and sericite. Somefeldspar in trace amounts may be authigenic in quartz-chlorite meta-morphic rocks.Amphibole

Detrital amphibole totally disappears from the sediments with depthin all of the holes. A fibrous amphibole-like mineral was observed bySEM (Plate 2, Fig. 1) infilling pores and replacing foraminiferal tests.Fibrous amphibole of similar appearance has been described previouslyin a low-temperature hydrothermally altered sediment sample dredgedfrom the Hess Deep (Rosanova et al., 1978).MicaDetrital mica (both biotite and muscov ite) is affected by chloriti-zation.Pyrite

Disseminated authigenic pyrite occurs as small cubic crystals,spherulites (framboids), and irregular particles, which may replaceorganic fragments in all clay and silty clay samples. Both diageneticand hydrothermal origins are possible for this type of pyrite; however,the concentration of disseminated pyrite in some thin sections seemstoo high for common diagenetic processes. The diagenetic pyritiza-tion may be accelerated by hydrotherm al activity. Pyrite of apparentlyhydrothermal origin occurs as crystalline aggregates in pore spaces,as small veins, and as incrustations around anhydrite crystals. Earlydiagenetic or hydrothermal pyrite also occurs as burrow fillings andreplaces foraminifers and diatoms.Large rectangular (crystal-like) or irregular pyrite segregationscomposed of fine-crystalline pyrite aggregates occur both in clay-stone and in siltstone layers. Their crystal-like rectangular shape maybe inherited from another mineral (anhydrite ?) that was replaced bypyrite, forming pseudomorphs.Anhydrite and Gypsum

Anhydrite and gypsum occur as crystalline aggregates infillingsmall fractures and caverns and as poikilitic inclusions, with sedimentincorporated into the skeletal crystals. Textural intercalations in the

sulfates indicate replacement of anhy drite by gypsum. Large segrega-tions (veins?) of euhedral elongated anhydrite crystals occur as radi-ated or parallel intergrowths, with pyrite infilling interstitial spacebetween the crystals. Authigenic forms of sulfates are shown in Plate2, Figures 2, 3, and 4.Carbonates

Carbonate minerals include biogenic foraminiferal tests and frag-ments, "unidentified" fragments, and crystalline calcite (which couldbe recrystallization products of biogenic carbonate), coarse-crystal-line (sparry) calcite cement, detrital grains, diagenetic calcite concre-tions, microconcretions and burrow-fills. Authigenic carbonate fillspore space. Calcite veins cut brecciated sedimen ts.Epidote

Epidote occurs both as detrital grains and as authigenic crystals.The latter occur in cem ent as euhedral crystals (Plate 2, Figs. 5 and 6)and as aggregates of parallel or radial crystals and rims around clasticgrains. In some cases epidote-clinozoisite seems to replace plagio-clase. It also corrodes crystals of pyrite. The epidote forms stringersand veinlets.Zeolites

Wairakite/analcime veinlets are composed of euhedral crystals(Plate 2, Figs. 8 and 9). Unidentified zeolite (clinoptilolite ?) and rareanalcime were detected in the coarse light fraction. They are eitherhydrothermal or diagenetic in origin.Ruffle an d Anatase

The titanium-bearing minerals rutile and anatase appear to be ofauthigenic origin. They fill interstitial space together with opaqueminerals (pyrite) and incrust small pores in some cases. We found alsosmall segregations of a finely crystalline titanium m ineral (anatase ?)in Hole 858D.Other Minerals

Barite is possibly authigenic. Garnet, zircon, sphene, and apatiteare likely to be authigenic in part. Zoisite or clinozoisite replacesplagioclase.SECONDARY MINERALS OF BASALTS

Secondary minerals in sills (chlorite, corrensite-like mineral, epi-dote, uralite, prehnite, quartz, albite, chalcedony, zeolite, leucoxene,sphene, and sulfides) were formed by hydrothermal alteration.Chlorites w ith different Fe/Mg ratios are the principal secondaryminerals in the igneous rocks recovered during Leg 139. Fe-richchlorites have low-intensity (001) reflections at 14 and high-inten-sity (002) reflections at 7 A (Fig. 4, Sample 139-857C-63R-1,50-52cm). Fe-Mg chlorites with Fe slightly predominating over Mg havean (001) reflection that is less intense than the (002) reflection (Fig.4, Sample 139-857C-59R-1, 20-22 cm). Mg-Fe chlorites have an(001) reflection higher than (002) (Fig. 4, Sample 857C-61R-2, 8-10cm). Chlorites may contain hydrated interlayers (rarely up to 10%).In this case, the (001) reflection moves to 14.5-14.7 in glycerine-saturated specimens.Corrensite-like m inerals are found toge ther with chlorites. Typicalreflections of the corrensite-like minerals are shown in Figure 5(Sample 139-857C-60R-1, 130-132 cm).

DOWNHOLE DISTRIBUTION OF MINERALS ANDZONATIONX-ray diffraction results are listed in Tables 1 and 2, immersionresults in Table 3, and petrographical and X-ray diffraction results for

116


5/19

3.54

4.72

14.0

3.54

7.08

14.1

14.1

3.54

Figure 4. X-ray diffraction patterns of chlorites from igneous rocks (specimensare air-dried). A. Fe chlorite, Sample 139-857C-63R-1, 50-52 cm; B. Mg-Fechlorite, Sample 139-857C-59R-1, 20*22 cm ; C. Fe-Mg chlorite, Sample139-857C-61R-l ,8-10cm.

MINERALOGY OF SEDIMENTS AND IGNEOUS ROCKS7.1

14.2

Figure 5. X-ray diffraction pattern of Mg-Fe chlorite and corrensite-likemineral (Sample 139-857C-60R-1, 130-132 cm , saturated by glycerine).altered basalts in Table 4. The mineralogical data show that all sec-tions at Sites 85 6,85 7, and 858 are divided into three main zones withsome variations in intensity of alteration and in complexity of themineralogical assemblages.

Site 856The Quaternary sediments recovered at Site 856 were divided byshipboard scientists into two lithostratigraphic units. Lithologic UnitI is represented by massive silty clay, 2.32 m thick. Unit II is charac-terized by alternating hemipelagic and turbiditic sediments that arevariously altered. Subunit IIA is weakly altered, Subunit IIB wa srecognized as a slump(?) deposit and is present in Hole 856B only,Subunit IIC is moderately indurated and altered, and Subunit IID ismoderately to well indurated and contains abundant sulfide minerals.

Hole 856AAlteration Zone I coincides with lithologic Unit I (0.00-2.32mbsf). Zon e II coincides with lithologic Subunits IIA and IIC (2.32 to113.22 mbsf).In Zone II the chlorite content gradually increases dow nhole, from56 % to 81% of the total fraction of clay minerals, while hydromica.decreases. Smectite is rare (Table 1, Fig. 6). Anhydrite and gypsumare present in trace amounts throughout the section. Quartz, feldspar,chlorite, and mica are the dominant detrital phases. Amphibole ofpossibly authigenic origin was detected by XRD (Table 2) in theuppermost part of the section (6.3 mbsf).The heavy-mineral fraction in Hole 856A is dominated by pyrite(Table 3). The pyrite crystals in many samples are oxidized, so thatFe-oxide particles con stitute up to 80% of the heavy fraction. Ep idoteoccurs both as detrital grains and as crystal aggregates. Siltstone

samples show weak recrystallization of quartz.Secondary minerals we re studied in a single sample of basalt. Thebasalt is weakly altered (Table 4) in a manne r similar to the overlyingsedimen ts. Olivine is replaced by iddingsite and talc (Table 5). Plagio-clase is replaced by prehnite and chlorite.Mineralogical data show that the terrigenous "background" sedi-ments in Hole 856A are affected by some hydrothermal alteration.Indicators of this weak alteration are traces of authigenic anhydrite,gypsum, pyrite, and amphibole; increase of chlorite content and dis-solution of biogenic carbonate downward; and weak recrystallizationof quartz below 70 mbsf.

117


6/19

V. KURNOSOVETA L.Unit Site 856A Unit Site 856B

100

Figure 6. Variations inclay mineral composition of the clay fraction, Site 856.1 = chlorite; 2 = mica; 3 = smectite; 4 = corrensite-like mineral; 5 = basalt.Hole 856B

The style of alteration in Hole 856B is the same as that in Hole856A, but more intense.Zone II (0.00 to 65.44 mbsf) coincides with lithologic SubunitsIIA, IIB, and IIC and includes among the authigenic minerals mainlyquartz, pyrite, chlorite, and anhydrite. Hole 856B contains a largeramount of pyrite and anhydrite than Hole 856A. Feldspar decreaseswith depth (Table 2). Hydrothermal dissolution of biogenic carbon-ates occurs at a much shallower depth in Hole 856B (below Core139-856B-1H, Section 1) than inHole 856A (below 55mbsf). In onethin section from Hole 856B (45.2 mbsf), silty sandstone is exten-sively carbonatized. Thecarbonate is developed aspatchy cement.The sandstone contains moderately abundant titanium-bearingminerals (rutile and anatase) that exhibit an authigenic appearance.They fill interstitial space together with pyrite. Other minerals occa-sionally present in trace quantities aregarnet, zircon, sphene, apatite,and barite, with rare olivine. Anunidentified zeolite (clinoptilolite ?)is present in the light fraction (Table 3),which occurs throughout thesection inboth holes.The slump(?) sediments (Subunit IIB) contain abundant chloritewith traces of the corrensite-like mineral. This mineral assemblagereflects more intense alteration than that in Subunit IIA.Zone III (70.80 to 120.51 mbsf) coincides w ith lithologic SubunitIID and includes among the authigenic minerals mainly pyrite, chlo-rite, and quartz. Disseminated pyrite and possibly other sulfides areabundant (Table 3).Pyrite occurs as large rectangular pseudomorphs.Authigenic quartz occurs asmosaic cement and fissure and cavity fill-ing. Feldspar israre orabsent (Table 2)andhasbeen replaced byquartzan d Fechlorite. Authigenic chlorite occurs incement (Table 1, Fig. 6).Authigenic hydromicaoccurs asradial aggregates. Anhydrite isabsent.

Site 857Lithologic Unit I is a Holocene to upper Pleistocene silty clay(hemipelagic sediments and turbidites). Thedegree of induration andhydrothermal alteration wasused by shipboard scientists to subdividethe Pleistocene turbidites (Unit II) into three Subunits: IIA, IIB andIIC. Sediments of Subunit IIAexhibit little induration. Sediments of

Subunit IIB are moderately to strongly indurated. Sediments of Sub-unit IIC are strongly indurated and hydrothermally altered, and areinterbedded with igneous rocks. Geochemically the composite se-quence drilled atSite 857 wasdivided into twozones with a boundarysomewhere near 400-450 mbsf.Mineralogical data distinguish three zones of hydrothermal altera-tion in sediments recovered at Site 857: Uppermost Zone I (0 to145-176 mbsf) with weakly altered sediments, Zone II (145-176 to400-450 mbsf) with moderately altered sediments, and lowermostZone III (400-450 to 926mbsf) with strongly altered sediments inter-bedded w ith sills.Alteration Zone I coincides with lithologic Unit I, Subunit IIA,and the uppermost part of Subunit IIB. Sediments from this zone arecharacterized by a high content of smectite (Table 1, Fig. 7).Detritalminerals include quartz and feldspar (Table 2).Amphibole (of possi-bly authigenic origin) was found by XRD in the uppermost sample(69 mbsf). Quartz and feldspar predominate in the light fraction, withminor mica and a high content of biogenic carbonate (Table 3). Thefeldspars are represented mainly by sodium plagioclase and potas-sium feldspar with minor andesine-labradorite.The carbonate m inerals, including biogenic foraminiferal tests andfragments, "unidentified" fragments, and crystalline calcite, whichcould be a recrystallization product of the biogenic carbonate, arepresent athigh percentages (up to 66%) to about 125mbsf (Table 3).Below this depth the carbonate content decreases sharply, indicating atransition from thecarbonate recrystallization and cementation zone tothe carbonate dissolution zone .The heavy fraction (Table 3) contains pyrite and iron oxides re-sulting from pyrite oxidation. G arnet, zircon, sphene, anatase, apa tite,pyroxenes, barite, anhydrite, and "black opaques" (ilmenite ?) occuroccasionally as traces.Zone IIcoincides with lithologic Subunit IIB andcontains amongauthigenic minerals mainly quartz, pyrite, chlorite, and anhydrite.This zone does not show any significant changes in the major ele-ments relative to "background" values (shipboard data). Themajor con-stituents are quartz and feldspar (Tables 2 and 3).Their relative propor-tions are almost constant, with quartz predominating over feldspar.The clay fraction is represented by Fe chlorite and less abundanthydromica; smectites areabsent (Table 1, Fig. 7).Authigenic carbon-ate and anhydrite infill pore space in sandy layers. An unidentifiedzeolite occurs occasionally (Table 3).Calcite and anhydrite were notidentified by XRD (Table 2). Weobserved large rectangular or irregu-lar pyrite segregations composed of finely crystalline pyrite aggre-gates, and mosaic quartz recrystallization.Thin section analyses from 155 to 414 mbsf indicate a generaldownhole increase inhydrothermal effects. M ore altered sandy layersor laminae are interbedded with less altered claystones. The hydro-thermal alteration is manifested by two main processes: pyrite pre-cipitation and quartz recrystallization. Chlorite with authigenic tex-tures is visible in the lower part of the zone, especially in sandstone-siltstone interbeds, where chlorite cements the detrital minerals.Zone IIIcoincides with lithologic Subunit IIC andcontains amongauthigenic minerals mainly pyrite, chlorite, epidote, and quartz.In this zone the clay fraction is totally composed of Fe chlorite(Fig. 7). Thin se ctions from 529 mbsf to the bottom ofHole 857D at927 mbsf show strong quartz ceme ntation and adownhole increase inauthigenic (hydrothermal) epidote-clinozoisite minerals. The quartz-ification is apparent even in fine-grained clayey siltstone (529.22mbsf), where mosaic quartz occurs as small lenses within a siltstonematrix containing very fine-grained epidote. Inbulk samples epidoteis abundant (Table 2).Epidotization becomes even more extensive downhole beginningat 829.7 mbsf. The sediment contains coarse-grained cubic crystals ofpyrite, rimmed by epidote. Epidote forms stringers and veinlets withsulfides, in laminated silt and clayey silt. Strong quartz ceme ntation isaccompanied by possibly authigenic mica. InSample 139-857D-28R-1,30-32 cm(839.2 mbsf) amineral wasdetected by XRD that may be

118


7/19

MINERALOGY OF SEDIMENTS AND IGNEOUS ROCKSTable 1. Results of X-ray diffraction of clay minerals in sediments.

Core, section,interval (cm) Depth(mbsf) Chlorite Hydromica Smectite CorrensiteFraction Unit (%) (%) (%) (%)139-856A-2H-3, 67-695H-7, 26-307H-4,133-1371IX-1,77-7913X-1, 108-11113X-4, 94-98139-856B-3H-1,108-1103H-2, 58-623H-2,* 58-623H-3, 92-963H-3,* 92-963H-4, 62-663H-4,* 62-6 63H-4,* 62-667H-4,46-497H-4,* 46-4 98H-2, 74-7613X-4, 84-8814X-2, 97-10 015X-4,73-75139-857A-2H-2, 8-102H-2,* 8-106H-3,40-42139-857C-2R-2, 24-253R-2,74-7610R-1, 72-7411R-1, 103-10512R-2,47-5015R-3, 74-7617R-2, 112-11624R-1,44-4827R-2, 81-8631R-2, 7-936R-2, 139-14336R-2,* 139-14341R-3,* 48-5043R-2, 20-2343R-2,* 20 -2351R-2, 75-7851R-2,*75-7865R-1, 12-1467R-1, 28-30139-857D-11R-1, 49-5116R-1, 36-3821R-1, 10-1224R-1, 50-5228R-1, 30-3229R-1, 9-1030R-1, 20-2134R-1, 26-3037R-1,* 42-43139-858A-1H-1,39^H3H-6, 37^14H-6, 23-274H-6,* 23-277H-2, 40-437H-2,* 40-4311X-CC, 22-2511X-CC,* 22-2514X-1* 6- 8

6.3739.3456.0386.97106.38110.7412.3813.3813.3815.2215.2216.4216.4216.4254.2654.2661.0496.44103.27115.6312.9812.9844.3058.2468.74124.82134.83145.47176.24194.52259.64286.41324.27348.99348.99378.38386.40386.40425.65425.65529.22548.68

676.69724.86773.80801.70839.20848.49859.10897.76926.920.3919.5429.1329.1351.8051.8073.6973.69101.06

333333113131231313311211311311113133131311111113111311212121

IIAIIC

IIB

IIC

IID

IIAIIAIIB

IIC

IIC

IIAIIC

5674778279817082719384949690676572868383312629345016852727582807781817779768281100100

100100100100100100100100100402161706063626677

26231821193018297166410333528141741717201719134162825182023191923211819

2817393040373834

tr

tr

135257514931718832

3262

Core, section,interval (cm)14X-1,* 6-814X-CC, 18-2016X4,7-917X-1, 32-3417X-1,* 32-3418X-2, 71-7318X-2,* 71-7320X-1,96-9820X-3, 88-9121X-2, 60-6221X-2,* 60-6221X-3,19-2121X-3,* 19-2124X-1,40-4231X-1, 30-3231X-1,* 30-32

139-858B-1H-1, 57-611H-4, 33-371H-4,* 33-372H-1, 33-352H-1, 33-352H-2, 75-772H-4, 68-702H-4,* 96-982H-4,* 96-982H-6, 46-482H-6,* 46-4 83H-1, 126-1285H-2, 17-195H-4, 14-185H-4,* 14-188X-1, 13-15

139-858C-1H-1, 112-1162H-2, 26-283H-1,* 100-1023H-6,48-505H-3,46-486H-6H-6H-:7H -7H -7H -7H -

,114-116,114-116>,46-48,16-19,* 16-191,115-118,* 115-1187H-2, 124-1277H-3, 123-1277H-3,* 123-1277H-4,10-148H-1, 12-148H-1, 28-3011X-1, 71-7314X-CC, 4-6139-858D-2H-3, 95-973P-1, 17-193P-1,* 17-193P-1, 17-194H-3, 65-694H-3,* 65-694H-4, 100-1044H-6, 124-1264H-6,* 124-1265H-CC, 36-38

Depth(mbsf)101.06101.18120.37130.22130.22141.81141.81159.96162.81170.70170.70171.60171.60198.00265.60265.60

0.574.834.837.537.539.4512.3812.6612.6615.1615.1617.9625.5728.5428.5432.83

1.125.2614.0020.9826.9634.1434.1434.9641.6641.6642.6542.6544.2445.7345.7346.1046.6246.7855.2184.2213.2518.9718.9718.9722.1922.1224.0427.2827.2828.66

Fraction21313123312121121131211121231211121121131113131311123122133

Unit

IIII

IVI

IIBIID

IIIB

IIC

IIIA

Chlorite(%)

79658095838475858389898988851412211015261025258383255tr

100282838386810010010010090959110010092875594959828686562555954505579

Hydromica(%)2135205161716251512171111111215

91333269trtr17

17

4017422332trtrtr1059trtr81345652

10323538454146504521

Smectite(%)

777546887965

32552039

tr

62

Corrensite(%)

9075757595100100

Notes: Clay minerals as 100%. 1 = fraction less than 0.001 mm; 2 = fraction 0.001-0.01mm; 3 = nonseparated clay fraction from bulk samples; tr = trace amounts; =not used in diagrams.

a Ba-silicate or amphibole. Extensive epidotization and quartzifica-tion was observed in samples from Cores 139-857D-29R through-37R (848 to 926 mbsf). Large skeletal(?) crystals of sulfide alsooccur in this interval. Radiated crystals of epidote were observed in afine-grained clayey interbed (897.8 mbsf) that has a high chloritecontent, based on XRD.Sulfide minerals, including pyrite and possibly others, occur alsoin veinlets along with anhydrite, quartz, and epidote. The veinlets insome cases cut parallel laminations of the turb idite.

We recognized the following secondary alterations of the igneousrocks recovered in Holes 857C and 857D (Table 4). Pyroxene is par-tially uralitized, with formation of fine-grained aggregates of leuco-xene and sphene. The primary plagioclase is partially albitized andreplaced by chlorite, epidote, and zeolite. Olivine is replaced by chlo-rite and leucoxene. G roundmass glass is chloritized. Veinlet mineralsinclude chlorite, leucoxene, chalcedony, sulfides, epidote, quartz, andalbite. Chlorite, epidote, quartz, and sulfide minerals fill pores andform amygdules.

119


8/19

V. KURNOSOV ET AL.Table 2. Results of bulk X-ray diffraction mineralogy of sediments, Leg 139.

Core, section,interval (cm) Depth(m) Unit Cor Anh Gyp Pyr Ep Cal W139-856A-2H-3, 67-695H-7, 26-307H-4, 133-1377H-4, 133-1371IX-1,77-7913X-1, 108-111

13X4 , 94-98139-856B-3H-1, 108-1103H-2, 58-623H-3, 92-963H-4, 62-666H-4, 95-977H-4,46-498H-2, 74-7612X4 , 106-11013X4,84-8814X-2, 97-10015X4, 73-75139-857C-3R-2, 74-7612R-2,47-5015R-3, 74-7617R-2, 112-11618R-2, 38-42

24R-1,44-4627R-2, 81-8631R-2, 7- 933R-2, 4-636R-2, 139-14341R-3, 48-5043R-2, 20-2351R-2, 75-7865R-1, 12-1467R-1,28-30139-857D-1R-2, 19-228R-1,5-711R-1, 49-5114R-1, 1-4*16R-1, 36-3817R-1,45-4821R-1, 119-12121R-2, 10-1222R-1,27-2924R-1,50-5227R-1, 13-1528R-1, 30-3229R-1.9-103OR-1,20-2131R-1, 13-1534R-1,26-3037R-1,42-43139-858A-1H-1, 39-413H-6, 37-414H-6, 23-275H-3, 32-3711X-CC, 22-2514X-1,6-814X-CC, 16-2016X-1,7-917X-1, 32-3418X-2, 71-7 32OX-1,96-982OX-3, 88-9121X-2, 60-6221X-3, 19-2323X-1,97-9924X-1,40-4231X-1.30-3239R-CC, 15-17139-858B-1H-1, 57-611H-4, 33-372H-2, 75-772H-4, 68-702H-4, 96-982H-6, 46-485 H 4 , 14-188X-1, 13-15

6.3739.3456.0356.0386.97106.38110.74

12.3813.3815.2216.4245.2554.2661.0487.1696.44103.27115.6368.74145.47176.24194.52203.48259.64286.41324.27333.94348.99378.38386.4425.65529.22548.68

583.19647.35676.69705.11724.86734.35773.39773.8782.07801.7829.73839.2848.49859.1868.63897.76926.920.3919.5429.1334.2273.69101.06101.18120.37130.22141.81159.96162.81170.7171.6188.87198265.6332.550.574.839.4512.3812.6615.1628.5432.83

IIA

IIC

IIC

IID

IIA

I1B

IIC

IIIA

IIC

IIII

IIBIID

120


9/19

MINERALOGY OF SEDIMENTS AND IGNEOUS ROCKS

Table 2 (continued).Co r e , section,interval (cm)

D e p th(m ) U n i t Am Cor Anh Gyp Pyr Ep C al W

139- 858C-1H- 1,112-1163H-6,48- 505 H - 3 , 46^86 H - 3 , 34-367H- 1,16-197 H - 1 , 115-1187 H - 2 , 124-1277 H - 3 , 123- 1278H-1,12-148 H - 1 , 28-3011X- 1, 71- 7314X- CC, 4- 6

139-858D-2 H - 3 , 95- 983 P - 1 , 17-194 H - 3 , 65-694 H - 4 , 100-1044 H - 6 , 124-1265 H - C C , 36-386X- 1, 25-27

1.1220.9826.9636.3441.6642.6544.2445.7346.6246.7855.2184.22

13.2518.9722.1924.0427.2828.6629.05

I + + + + ++ + + + ++ + + ++ + + ++ + + + +I I B + + + + ++ + + + ++ + + + ++ + + + ++ + + + +II C + + + + ++ + + + +

I + + + + ++ + + + ++ + + + +H A + + + + 4+ + + ++ - H - +

D + + + +N o te : + + + + = do m i n a n t , + + + = major, + + = minor, + = race; * = talc ( trace) ; Q = quartz, F = feldspars, Am = amphibole,

Cor = corrensite-like mineral, Anh = anhydri te, Gyp = gypsum, P y r = p y r it e , Ep = ep i d o t e , Cal = calcite, W = wairacite.Chlorite with Fe predominating over Mg is the most common

alteration product . Sometimes, especially in the lower par t of the igne-ous complex, corrensite- like minerals occur together with chlorite.O n the whole Zone III sediments and the igneous rocks interbedded

with them have undergone alteration in the greenschist facies of m e t a -morphism. The sediments are chloritized (Fe chlorite), epidotized, andquartzified. Presumably there are newly- formed feldspars in thesesediments. The igneous rocks are extensively chloritized, and epidote,quartz, albite, and uralite also occur.

Site 858Lithologic U n i t I (Davis, M ot t l , Fisher, et al., 1992) is composed

of silty clay of H olocene to late Pleistocene age. Lithologic U n i t II ischaracterized by alternating hemipelagic and turbiditic sediments andis generally correlative to lithologic U n i t II at the previous Leg 139sites. At Site 858, lithologic U n i t II is divided into four subunits.Subunit I I A is nonaltered to weakly altered, Subunit I IC is moderatelyto well indurated and altered, and Subunit II D is completely silicified,commonly fractured, and contains abundant authigenic silica andsulfide minerals.

These three subunits are lithologically similar to Subunits IIA,I I C , and IID at Site 856. Subunit IIB is a brecciated, anhydrite, andcarbonate- rich sequence that contains semimassive sulfide. SubunitII B is in a stratigraphic position similar to that of the slumped (?)Subunit IIB at Site 856. Lithologic U n i t II I consists of metalliferous,oxidized surficial sediments. Lithologic U n i t IV consists of strata-bound semimassive sulfide. Lithologic U n i t V consists of mafic ex-trusive rock recovered at the base of Holes 858F and 858G.Hole 858A

Mineralogical data distinguishes three zones of hydrothermal al-terat ion in sediments from H ole 858A.Z o n e I (0- 21.40 mbsf) coincides with lithologic U n i t I and con-tains abundant auth igenic smectite. The zone is a smectite- bearing(Fig. 8), or smectite- dominated (19.5 mbsf) hemipelagic silty clay.Z o n e II (21.40 to 315 mbsf) coincides with lithologic Subunits IIAan d IIC and contains among auth igenic minerals mainly pyrite, chlo-rite, an d anhydrite, as well as calcite in the upper part of the zone, andepidote and quartz in the lower part of the zone. Mineralogical datashow that Zone II is divided in to Subzones IIA (21.40 to 120 mbsf)and I I B (120 to 315 mbsf).

Subzone IIA is characterized by detrital quartz predominatingover feldspars, based on bulk XRD (Table 2) and by approximatelyuniform ratios of mica (40%) to Fe chlorite (60%) in the clay fraction(Fig. 8). Trace amounts of anhydrite occur throughout and gypsumalso was found (Table 2).

This subzone is characterized in shipboard descriptions as aninterval with carbonate concretions. These occur together with anhy-drite and pyrite concretions. In th in sections we observed recrystal-lized foraminifers covered by rusty Fe- oxide films, some of whichlook like micronodules in the coarse fraction. C alcite micronodulesoccur as detrital grains and as ellipsoidal nodule- like segregations, thelatter being possibly of authigenic (hydrothermal) origin. Zoisite orclinozoisite replaces sodium plagioclase grains.

T h e coarse heavy fraction (Table 3) contains high percentages ofpyrite and Fe- oxides; epidote, barite, and garnet are common trans-parent minerals. In the light fraction downhole trends of increasingquartz and decreasing feldspar were observed. Both muscovite andbiotite decrease downward.

In thin sections, sandy layers are seen to contain amphibole,quartz, and feldspar. The sediment is partially cemented by carbonate.Carbonate also occurs as recrystallized foraminiferal tests. Epidote-clinozoisite occurs commonly in the cement. Leucoxene is present.

Subzone IIB as a whole is characterized by a high quartz contentt h a t increases downward, by variable feldspar c ont e n t , and by chloriteapparen tly prevailing over mica. Most significant, however, is thehigh and variable anhydrite and gypsum content (Table 2 ). Fe chloriteprevails strongly over hydromica in the clay fraction, with someoscillations in their relative amounts (Table 1, Fig. 8).

Th e interval below 175 mbsf contains low concentrations of sul-fates. Quartzification (quartz cementation) is more extensive. Carbon-ate is absent. Chloritization of mica is accompanied by pyritization,leading to formation of chlorite aggregates with abundant dissemi-n a ted finely crystalline pyrite.Z o n e II I (315 to 332.6 mbsf) coincides with lithologic Subunit II Dan d contains among authigenic minerals sulfide, chlorite, quartz, andwairakite- analcime veinlets.Hole 858B

Three zones of hydrothermal alteration were distinguished in H ole858B. The semimassive sulfides are not included in our study.Z o n e I (0 to about 10 mbsf) coincides with lithologic Units I andI I I , characterized by the presence or even domination of smectites in

121


10/19

V. KU RN OSOV ET AL.

Table 3. Co arse fraction (0.1- 0.05 mm) min eralogy.A. Heavymnerals.

Core, sectioninterval (cm)

139-856A1H-1, 30-33*2H-3, 67-69*2H-2 , 45-494H- 6, 6-10*5H-7, 26- 306H-5, 84-888H-4, 82-85*9H-3, 56-5910X- 1, 55-5713X- 3, 93-97

139-856B-1H-1,33-372H-3, 106-1103H-1 , 108-1123H-2 , 58-623H-3 , 92-963H-4, 62- 664H- 4, 67-686H-4, 95- 977H-4, 46-499X- 1,55- 5912X- 4, 106-

11013X- 1,69-73

139-857A2H-2 , 8-106H-3, 40-447H4, 30-34139-857C-

1R-1, 103- 1052R-2, 24-251 OR- 1,72- 7415R-3, 74- 7617R-2, 112 -

116139-858 A

3H-6, 37- 414H-6, 23- 275H-3*, 32-377H-2* 404311H-CC, 22-2514X-1CC, 18 -20

17X- 1, 32- 34139-858B-

1H-4*, 33- 372H-6, 46-485H-4, 14-18

139-858C-1H-1, 112-1162H-2 , 26-285H-3,46-486H-2,46-487H-1 , 16-197H-1, 115-1187H-2 , 121-1277H-4, 10-1414X- 2,4-6

139-858D-4H-3 , 65-694H-6, 124-126

Py

1.45.422.2.

76.452.31.211.960.472.4

76.092.360.912.695.798.595.583.490.989.580.190.1

88.096.754.6

76.374.069.713.323.9

91.681.8 n89.389.67.85.5

86.998.2

92.989.596.197.592.286.895.990.196.3

94.791.4

Io x

3.13.45.96.38.740.61.2

79.931.69.8

18.92.22.72.82.8-1.814.64.62.1

14.36.3

4.20.728.4

1.213.613.81.3

2.33.4.

7.811.9

5.4_9.4-

2.74.61.4-5.110.6.

-

1.81.8

Ep

-

_

13.20.851.45.46.88.7

7.61.5.0.20.90.61.41.4.

0.50.81.7

1.21.5

13.70.92.02.049.8

_0.50.55.63.1

36.2_

0.6-

1.51.51.70.2.2.2

2.5

0.2

Gr

-

-0.3.

0.20.40.30.30.5

_

_

-

-

-

-

-0.70.91.61.60.6

0.2-

0.40.6_0.6

0.7

15.4

_

-

_

-

_

0.3

0.4

Zrk

0.2

0.20.80.3-

-

-

-0.3.0.90.80.8-

-

1.0

0.6

_

_

_

3.2

-

_

_

-

-

Sph

-

-

-

0.20.8-

-

-

-

_

-

-

-

-

-

3.8

-

1.00.2

1.6

_

-

-

-

AP

-

0.50.4-

0.30.30.3

-

4.2

0.3

-

-

0.61.33.0

-

-

-

8.5

-

0.5-0.7-

-0.6-

-

Px

-

-

-1.60.30.3-

-

-0.20.30.6-

-0.30.8-

0.40.21.0

-

-

0.6-

_

-

0.5

-

-

0.30.7-

-

-

-

-

-

-

-

0.9

Am

-

-1.0-

-1.50.30.3-

-

-3.7-

-

-

-

-

-1.10.5-

2.50.63.0

5.4-

3.2-0.8

1.71.1-

-

0.7-

-

-

0.91.0-

-

-

-

-

-

-

-

-

Bar

-

-

-

-

-

-

-1.0-0.5

2.5-

-

-

-

-

-0.3-0.8

-0.74.5

0.2-

-3.42.5

0.8-2.1

0.21.71.03.30.30.2

0.61.0-0.20.70.43.12.0-

3.10.2

Chi

-

-

-

-

0.2-

-

0.3-0.3

-

-

-

-0.60.3-

-

1.23.0

0.25.8-1.50.4

-

-

-

0.2-

1.0-

-

-

-

-

-

-

-

-

-

-

-

-

Anh

-

-

-

-

-

-0.9-

-0.4-

-

-

-0.7-

0.5

0.6

-

-

-1.9-

-

-

0.3-

-

7.5-

-

-

-

-

-

-

-

-

-

-

-

th e clay fraction (Table 1, F ig. 9). The upper 2 m is "metalliferous"clay (green, greenish gray, greenish black to dark bluish green, withbrownish yellow oxidized surface layer). We observed nontroniteaggregates in a smear-slide; smectite dominates in the clay fraction(Fig. 9). In smear slides the smectitic clay contains detrital quartz,feldspar, and minor mica, with rare epidote- clinozoisite and leu-coxene, th e latter in small aggregates with rutile. Finely crystallinedisseminated anhydrite and pyrite are present in minor amounts.

Z o n e II (10 to about 30 mbsf) coincides with lithologic Units I(lower part, below U n i t IV), IV, and Subunit I I B and contains amongauthigenic minerals mainly pyrite, corrensite-like mineral, chlorite,

and anhydrite. Bulk XRD data shows high quartz content and traceamounts of feldspar (Table 2). The major change in clay mineralogyis thedisappearance of mica and the appearance of Mg- rich mixed-layer smectite/ chlorite (corrensite- like mineral) (Table 1, Fig. 9). Inthin section we observed authigenic well- crystallized chlorite (orcorrensite) clay with abundant pyrite, both disseminated finely crys-talline and as aggregates of large crystals (pseudomorphs after anhy-dri te?) . A radiated aggregate of elongated anhydrite crystals includeseuhedral cubic pyrite and is partially replaced by gypsum.

Z o n e II I (30 to 38.6mbsf) coincides with lithologic Subunit I I D .A single Sample 139-858B- 8X- 1,46^8 cm (32.83 mbsf) represents


11/19

MINERALOGY OF SEDIMENTS AND IGNEOUS ROCKSTable 3 (continued).B. Light minerals.

Core, section,interval (cm )

139-856A-1H-1, 30-332H-3, 67-692H-2,45^94H-6, 6-105H-7, 26-306H-5, 84-888H-4, 82-859H-3, 56-5910X-1, 55-5713X-3, 93-97

139-856B-1H-1,33-372H-3, 106-1103H-1, 108-1123H-2, 58-623H-3, 92-963H-4, 62-664H-4, 67-686H-4, 95-977H-4,46-499X-1, 55-5912X-4, 106-11013X-1, 69-73139-857A-2H-, 8-106H-3,40-447H-4, 30 -34139-857C-1R-1,103-1052R-2, 24-2510R-1, 72-7415R-3, 74-7617R-2, 112-116139-858A-3H-6, 37-414H-6, 23-275H-3, 32-377H-2, 40-4311H-CC, 22-2514X-1CC, 18-2017X-1, 32-34139-858B-1H-4, 33-372H-6, 46-485H-4, 14-18139-858C-1H-1, 112-1162H-2, 26-285H-3,46-486H-2,46-487H-1, 16-197H-1, 115-1187H-2, 121-1277H-4,10-1414X-2,4-6

Qz

39.17.49.25.49.11.136.243.245.735.57.117.34.75.15.740.813.110.342.341.312.6

27.78.216.318.6

18.216.022.226.144.0

18.014.211.27.338.612.831.716.762.34.25.66.558.528.817.55.9.12.366.0

Fs

11.24.31.82.72.60.817.611.521.019.03.47.61.56.03.110.22.64.416.013.23.1

11.27.97.79.3

19.810.611.88.423.2

10.88.14.42.718.37.216.16.415.81.24.01.515.36.99.94.4.4.114.7

Bi

2.13.4.1.6-0.90.40.76.1

1.10.41.61.42.61.01.51.53.72.30.92.215.83.84.31.52.6-2.4

3.40.41.0-1.4-1.01.90.41.20.7.0.62.41.70.90.70.9

Mu

2.83.40.50.40.9.1.45.1.1.10.41.42.01.60.55.21.21.70.4

0.61.07.01.61.00.31.53.1

0.91.11.42.2

-3.30.80.92.23.31.71.51.32.4

Chi

0.72.3

0.30.41.40.60.40.21.00.40.71.40.31.0

1.30.7

1.50.3-

0.4_

Anh__

__

_

_

-

_1.1

1.5-

_1.1

8.00.5

-

_

0.2-

Gyp

-

-0.2-

0.40.3_

-

1.6-

-

--

-0.2

-

-

-

Ca

1.446.057.383.042.197.34.94.215.70.681.13.617.26.24.74.012.61.51.22.02.7

1.863.45.546.6

2.339.226.510.32.0

43.651.558.570.63.22.50.34.32.22.3

85.291.31.30.9-4.53.10.21.6

C A .

37.021.22.02.644.20.434.139.119.432.1

4.267.511.853.780.438.969.774.537.235.580.557.513.017.815.817.327.928.322.722.2

18.114.223.510.816.447.240.354.815.089.2

-14.150.666.373.894.576.114.4

Ze

1.01.10.3----0.80.50.6-1.80.4-1.71.2-1.5-0.9--

1.0-0.5-1.5---

----0.5-

1.3--0.6-0.4---

-Notes: = minerals counted without separat ion into heavy and l ight fractions . Py = pyri te , Iox = iron

oxides , Ep = epidote , Gr = garnet , Zrk = zircon, Sph = sphene, Ap = apat i te , Px = pyroxenes ,A m = amphibole , Bar = barite, Chi = chlorite, Anh = anhydrite. Qz = quartz, Fs = feldspars, Bi= biot i te , Mu = m u sc o v i t e , Chi = chlori te , Anh = anhydri te , Gyp = gypsu m, Ca = calci te (mainlybiogenic) , but w i th minor sparry calci te , C. A. = un ident i f ied c lay aggregates , Ze = zeol i tes . Otherrare minerals not included in the table are (heavy fraction): tremolite and/or actinolite, blackopaques , ol ivine, rut i le , l eucoxene, carbonate(?) ; ( l ight fract ion): chalcedony, opal A (s i l i ceousmicrofoss i l s ) , volcanic glass , talc(?). - = mineral not found.

the zone and is a chlorite-quartz metamorphic rock according to bulk Hole 858CXRD (Tables 1 and 2) and thin section description. Th is is consistentwith the shipboard XRD data, which show abundant quartz and rather Three zones of hydrothermal alteration can be distinguished basedhigh chlorite without other minerals. The clay fraction in our sample is on different authigenic (hydrothermal) m ineral associations.composed of pure Fe chlorite (Table 1, Fig. 9). In thin section the rock Zone I (0 to about 20 mbsf) coincides approximately with litho-is composed of elongated quartz crystals with interstitial chlorite. stratigraphic Unit I and is composed of hemipelagic silty clay with up

123


12/19

V. KURNOSOV ET AL.Table 4. Brief petrograp hic description of analyzed igneous rocks and secondary minerals, Leg 139.

Core, sectioninterval (cm) Depth(mbsf) Description139-856A-14X-1, 34-38 114.84 Pyroxene microdoleritic basalt. Phenocrysts: clinopyroxene (up to 1 mm) sparse olivine. Groundmass: non-oriented lathes and microliths ofplagioclase (0.2-1 mm) - labrador (A n55). Among microliths there are individuals up to andesine (A n35). Clinopyroxene in interstices. Opaques:magnetite, leucoxene, spinel, chalcopyrite.Alteration is low. Olivine is replaced by iddingsite and talc. Plagioclase and clinopyroxene are partially replaced by prehnite and chlorite. Zeolite ispossibly formed on the plagioclase.

471.30 Plagioclase-pyroxene dolerite, fine-grained with ophytic texture Alteration is strong (up to 50% of rock is altered). Pyroxene and glass of groundm assare chloritized. Veinlets: chalcedony in center is boardered by chlorite, leucoxene, sulfides.XRD data: Mg-Fe chlorite with rare hydrated interlayers; quartz as trace.473.17 Gabbro-dolerite medium-grained with gabbro-ophytic texture. Plagioclase (0.3-1 mm) - labrador (An 62). Clinopyroxene (augite) up to 0.5 mm indiameter.Alteration is moderate. About 10% of rock is chloritized. C hlorite and leucoxene replace glass in groundmass.XRD data: Fe-chlorite with rare hydrated interlayers; quartz as trace.

60R-1, 130-132 482.10 Dolerite blastoporphyritic fine-grained with ophytic texture. Rare euhedral sceleton sulfide crystals (up to 3 m m). Groundm ass: laths of plagioclase(up to 1 mm) - A n ^ ^ ; about 50% of rock is unhedral clinopyroxene (augite or Ti-augite), located in interstices between laths of plagioclase.Alteration is strong (about 50% of rock is altered). Pyroxene is partially uralitized and chloritized, earthy aggregates of leucoxene and grains of spheneoccur in chloritized matrix; pyrite is also common.XRD data: Mg-Fe chlorite with rare hydrated interlayers, corrensite-like mineral (minor); quartz and amphibole as traces.

139-857C-59R-1,20-22

59R-2, 57-59

61R-2, 8-10 491.94 Pyroxene hyalopilitic basalt.Alteration is moderate. B rown glass is replaced by chlorite, traces of zeolite.XRD data: Fe-Mg chlorite with rare hydrated interlayers; quartz and amphibole as traces.62R-1, 70- 72 500.70 Congo-dolerite medium-grained with ophytic texture. Clinopyroxene (augite), plagioclase (labrador). Opaques: Ti-magnetite.

Alteration is strong (about 50% of rock is altered). Pyroxene is replaced along cracks by chlorite and uralite. Patches about 1 mm in diameter arecomposed of fine-gr ained quartz, chlorite, leucoxene and in some cases epidote. Veinlets: epidote (in zolbandes), chlorite and chalcedony (in center).XRD data: M g-Fe chlorite with about 10% of hydrated interlayers; corrensite-like mineral (minor); quartz and amphibole as traces.62R-2, 40^4-2 501.88 Congo-dolerite (the same as in Sample 62R-1, 70-72 cm). Opaques:Ti magnetite and sparse blastoporphyritic sulfide crystals (up to 2 mm).Alteration is strong (about 50% of the rock is altered by chloritizationand sulfidization). Leucoxen e replaces Ti-magnetite. A ggregates of quartz grainsand sulfidization). Leucoxene replaces Ti-magnetite. Aggregates of quartz grains occur (0.1 mm ). Plagioclase is partially replaced by albite.XRD data: Mg-Fe chlorite with about 10% of hydrated interlayers; corrensite-like mineral (minor); amphibole as trace.63R-1, 50-5 2 510.20 Congo-dolerite (the same as in Sample 62R-1, 70-72 cm).Alteration is strong (about 25% of rock is chloritized). Chlorite and epidote replace clinopyroxene. M icrocracks are infilled by chlorite and epidote.

Hydrogarnet, leucoxene, sphene were found in chlorite pseudomorphs alter pyroxene.XRD data: Fe chlorite; amphibole as trace.64R-1, 18-20 519.58 Dolerite medium-grained with ophytic texture.Alteration is moderate (about 25% of rock is chloritized). Chloriter eplaces clinopyroxene, plagioclase and glass. Uralite and chlorite replaceamphibole.XRD data: Mg-Fe chlorite; amphibole as trace.68R-2, 69-71 560.19 Basalt ophyric, with vitrophyric texture.Alteration is moderate, groudm ass glass is chloritized.XRD data: Fe-chlorite; quartz and amphibole as traces.

139-857D-3R-2, 86-88

4R-2, 53-56

8R-1, 5- 7

601.63 Plagioclase dolerite porhyric with ophytic texture. Phenocrysts: plagioclase (3-5 mm) up to 20%; clinopyroxene (0.5-2 mm ); olivine. Someplagioclase grains are sharply zoned: with bitovnite in center; labrador (An65) - in periphery. Opaques: Ti-magnetite, sphene.Alteration is moderate (about 10% of rock is chloritized). Pyroxene and plagioclase are replaced by chlorite and leucoxene. Pseudomorphs of chloriteand leucoxene replacing pyroxene and olivine. Epidote and albite also occur.XRD data: Mg-Fe-chlorite; quartz as trace.610.93 Olivine-plagioclase basalt porphyritic with microlitic texture. Groundmass: m icrolites of plagioclase and pyroxene, glass.Alteration is moderate. Chlorite replacing olivine, pyroxene, and glass. There are chlorite and aggregates of small quartz grains, sulfides in interstices.XRD data: Fe chlorite; quartz and amphibole as trace.647.35 Plagioclase basalt with pilotaxitic texture. Plagioclase-labrador (An56) is about 20%. Gro undmass: microlites of plagioclase, volcanic glass withdisseminated opaque mineral (magnetite ?).Alteration is moderate (chloritization). There are quartz, albite, andsulfides (in center) in veinlets.XRD data: Mg-Fe chlorite with rare hydrated interlayers.

12R-1, 106-108 686.96 Dolerite fine-grained aphyric with doleritic texture. Plagioclase-labrador (An55) 0.5-0.7 mm, clinopyroxene-augite (0.1-0.2 mm). Opaques: Ti-magnetite, leucoxene.Alteration is low (chloritization is about 5% of rock). Chlorite replaces groundmass glass and pyroxene.XRD data: Fe chlorite with rare hydrated interlayers; quartz as trace.12R-2, 17-19 687.47 P yroxene basalt with intersertal texture.Alteration is moderate. Chloritization of glass and pyroxene.XRD data: Fe chlorite with rare hydrated interlayers; quartz and amphibole as traces.17R-1, 45- 48 734.35 Dolerite fine-grained aphyric amygdoloidal with ophytic texture.Clinopyroxene (50%), plagioclase (50%). Alteration is strong (about 30% of rock ischloritized). Plagioclase is replaced by albite, chlorite and epidote. Pyroxene is replaced by chlorite. Earthen aggregates of leucoxene and smallgrains of sphene replacing pyroxene. Amygdules (1-3 mm, about 40% of rock) are filled by aggregate of chlorite (clinochlore). In the central part ofsome amygdules there is epidote.17R-3, 72- 74 737.56 Olivine-pyroxene basalt amygdoloadal. Opaques: pyrite, Ti-magnetite.Alteration is strong (about 30% of rock is chloritized). There are chlorite and epidote in am ygdules. Zoiside also presents. Pyroxene is partiallyreplaced by uralite. Ti-magnetite is partially replaced by sphene.XRD data: Fe chlorite with rare hydrated interlayers; quartz and amphibole as traces.

124


13/19

MINERALOGY OF SEDIMENTS AND IGNEOUS ROCKSTable 4 (continued).

Core, sectioninterval (cm) Depth(mbsf) Description18R-2, 84-85 745.86 Gabbro-dolerite coarse-grained with poikilophyric texture. Clinopyroxene (up to 3 mm) with inclusions of plagioclase (labrador-An 65). In intersticesamong laths of plagioclase there are sometimes small grains of orthoclase with abundant needle-shaped ap atite and with grains of leucoxene,epidote,uralite.Alteration is strong. Chlorite and uralite replace pyroxene. Leucoxene, e pidote, sulfides.XRD data: Fe chlorite; amphibole as trace.20R-1, 64-6 6 763.14 Gabbro-dolerite (same as in Sample 18R-2, 84-85 cm).

Alteration is strong (about 30% of rock is chloritized). Orthopyroxene is replaced by chlorite and uralite. Chlorite infilling numerous m icrocracks.Sulfides,XRD data: Fe-Mg chlorite with about 10% of hydrated interlayers, corrensite-likemineral (minor); quartz and amphibole as traces.23R-1, 57-59

25R-1, 73-75

33R-1, 81-83

36R-1, 3-5

139-858F-26R-1,93-95

27R-1,25-27

28R-1, 43-45

139-858G-1R-1, 14-16

4R-1,35-37

7R-1,32-34

14R-1, 10-12

16R-1, 75-77

792.07 Dolerite medium-grained blastoporphyric with ophitic texture. Plagioclase 0.5-1 mm (labrador An55) about 50%. Clinopyroxene (augite or Ti-augite)about 50% . Opaques: sphene, leucoxene, Ti-magnetite, sulfides.Alteration is mode rate. Chlorite replacing pyroxene and glass in groundmass, infilling pores and cracks. Sphene and Ti-augite are replaced byleucoxene. Sulfides infilling pores and cracks.XRD data: Fe chlorite; quartz and am phibole as trace.811.53 Dolerite (same as in Sample 23R-1, 57-59 cm).Alteration is moderate (about 20% of rock is chloritized). Clinopyroxene is replaced by chlorite. Plagioclase is replaced by chlorite, epidote, albite.Sphene and Ti-augite are replaced by leucoxene.XRD data: Fe chlorite with rare hydrated interlayers; quartz and amphibole as traces.888.61 Dolerite (same as in Sample 23R-1, 57-59 cm).Alteration is moderate (about 25% of rock is chloritized).XRD data: Fe ch lorite; quartz and amphibole as trace.916.93 Microdolerite aphyric with microdoleritic texture. Plagioclase 0.1-1 mm (labrador An 52_55). Clinopyroxene 0.1-0.3 mm.Alteration is moderate (about 30% of rock is chloritized). Plagioclase and pyroxene are chloritized and leucoxenized. Plagioclase is replaced by albiteand zeolite.XRD data: Fe chlorite; quartz as trace.259.53 Olivine-pyroxene basalt porhyric, with hyalopilitic, partially with vitro-phyric texture, vesicular.Alteration is low. Olivine, pyroxene and plagioclase are partially replaced by chlorite. Chlorite infilling pores (0.3-1.5 mm in diameter) and cracks.XRD data: Fe-Mg chlorite with about 10% of hydrated interlayers corrensite-like mineral; quartz as trace.268.05 Basalt (same as in Sample 26R-1, 93-95 cm).Alteration is low (same as in Sam ple 26R-1, 93-95 cm).XRD data: Fe-Mg chlorite with about 10% of hydrated interlayers; corrensite-like mineral; quartz and amphibole as trace.277.93 Olivine-plagioclase basalt porphyric, with hyalopilitic texture.Alteration is low. Chloritization of phenocrysts and groundmass.XRD data: Fe-Mg-chlorite with about 10% hydrated interlayers; corrensite-like mineral.276.94 Olivine-plagioclase basalt slightly porphyric with microdoleritic texture.Alteration is moderate. Chlorite replaces glass, partially replaces plagioclase and pyroxene; olivine is totally altered. A lbite replace plagioclase.XRD data: Fe-M g chlorite with about 10% of hydrated interlayers;corrensite-likemineral (minor).306.15 Olivine-plagioclase basalt with hyalopilitic-porphyric texture. Opaques:rare sulfides.

Alteration is moderate. Phenocrysts and glass in groundmass are chloritized.XRD data: Fe-Mg chlorite with about 10% of hydrated interlayers; corrensite-like mineral.335.12 Basalt (same as in Sample 4R-1, 35-37 cm).Alteration is moderate. Phenocrysts and glass in groundmass are chloritized.XRD data: M g-Fe chlorite with about 10% of hydrated interlayers; corrensite-like mineral (minor).403.70 Plagioclase basalt porphyric amygdoloidal with microdoleritic texture.Alteration is moderate (about 15% of rock is chloritized). Chlorite replacing pyroxene and infilling amygdules. Opal, chalcedony, and albite partiallyreplace plagioclase.

XRD data: Mg-Fe chlorite with rare hydrated interlayers; quartz and am phibole as trace.423.65 Dolerite medium-grained amygdules with ophyric partialy poicilophitic texture. Lathes of plagioclase 0.5-1 mm, clinopyroxene.

Alteration is strong (about 50% of rock is chloritized). Chlorite and leucoxene replace pyroxene. Albite and epidote replacing plagioclase. Am ygdules( 0 . 5 ^ mm) are infilled by c hlorite, in other cases - by chlorite, epidote and quartz. Glass in groundmass is replaced by chlorite and epidote.XRD data: Fe chlorite; quartz and am phibole as trace.

to 25 % carbonate. The carbonates include biogenic calcite (stronglyrecrystallized foraminifers and nannofossils), diagenetic calcite con-cretions, microconcretions, and burrow-fills. Dolomite was found inCore 139-858C-2H by shipboard XRD. The biogenic carbonate pre-dominates in the c oarse fraction (Table 3), constituting 85% to 9 1% inthe light fraction. Bulk XRD data show somewhat lower quartz andfeldspar contents than in the sediments below (Table 2) ; quartz prevailsover feldspars. Pyrite, which is commo n in smear slides, is "rare."The coarse heav y-mineral fraction (Table 3), however, is stronglydominated by pyrite (84%-86%) that is partially oxidized. T he rest iscomposed of iron oxide minerals, hornblende, epidote, apatite, andpossibly authigenic barite. Quartz and feldspar occur in almost equalquantities in the light fraction, which is dominated by biogenic and

partially authigenic carbonates. We found a zeolite (weak reflectionsof clinoptilolite(?) are recorded by XRD analyses). The clay fraction(Table 1, Fig. 8) is composed of Fe chlorite, hydromica, and smectite.The latter may be authigenic, precipitated from the hydrothermallyaffected ocean bottom waters.Zone II (20 to about 54 mbsf) coincides with lithologic Subunit IIBand contains among authigenic minerals quartz, pyrite, chlorite, andanhydrite. It is highly variable in mineralogy, demonstrating ratherstrong hydrothermal effects. T he sediments are well indurated in thiszone, especially in the coarse-grained turbiditic interbeds, which arecemented by carbonate and/or by gypsum and anhydrite. Pyrite alsoserves as a cem ent and authigenic Mg-Fe chlorite is likely an impor-tant factor causing induration of the clay.

125


14/19

V. KURNOSOV ET AL.Unit Site 857A,C,P Table 5. Microprobe analyses of secondary m ineralsin igneous rocks, Leg 139.

1 0 0 0 -20 60

Percent100

Figure 7. Variations in clay mineral composition, Site 857. 1 = chlorite; 2 =mica; 3 = sme ctite; 4 = basalt.Zone III (54-91.6 mbsf) coincides with lithostratigraphic SubunitIIC. It is characterized by abundant quartz, high chlorite and pyrite,and low mica and epidote. Sulfates and carbonates are absent.

Hole 858DThe degree of hydrothermal alteration in Hole 858D increasesdownhole as one secondary mineral association replaces another.Although g radual, these changes in mineralogy define three zo nes.Zone I (0 to about 18 mbsf) is represented by nonlithified hemi-pelagic silty clay rather weakly affected by hydrothermal alteration.The zon e coincides w ith lithostratigraphic Unit I and is similar to theupper zon es in other holes at this site. The calcite content is rather highin samples from Core 139-85 8D-1H-1, but decreases to trace amountsin Cores 1H-2 through 2H-3. Dolomite was identified in Core 2H.Epidote, anatase, pyrite and barite were detected in trace amou nts. Theclay fraction (Table 1, Fig. 9) is dominated by smectite (above 60%)with minor Mg-Fe chlorite (about 30%) and low mica content.Zone II (18 to approximately 25 mbsf) is a well-developed zone ofsulfate precipitation, as indicated by high anhydrite and gypsum con-

tents. Pyrite also occurs on XRD records, as well as traces of epidote-clinozoisite, ankerite (?) and titanium-bearing m inerals (Table 2). Thezone is characterized by increased qual i f icat ion , indicated by highquartz content and decrease in feldspars.Transition to Zone III at about 25 -28 mbsf is marked by disappear-ance of feldspars and sulfates and a decrease in mica content. Epidote-clinozoisite, rutile, anatase, and barite were detected in thin sectionsand smear slides. Pyrite is present. Quartzification is apparent.Zone III (25 -28 to 40.7 mbsf) coincides with lithologic SubunitIID. A quartz-wairakite vein was detected from a metamorphosedsiltstone. The vein is composed of euhedral wairakite and quartz withminor sphalerite. According to shipboard study such veins are com -

C o m p o n e n tS i O 2T i O 2A1 2O 3C r 2 O 3FeOMnOMg OCaONa pK2OTo t a l

15 5 . 8 10.01

0.110 . 3 27.820 . 0 02 8 . 0 00.110.000.009 2 . 1 8

22 8 . 4 00 . 0 018.420 . 0 02 6 . 8 30.3317.16

0 . 1 60.130 . 0 09 1 . 5 6

37 0 . 6 00.012 1 . 0 50.000.000.000.00

1.189.350 . 0 0101.60

43 0 . 7 40.0017.960.222 2 . 3 40.2719.78

0.180.000 . 0 09 1 . 5 0Note : 1 = t a l c r e p l a c i n g o l i v i n e ( Sa m p l e 8 5 6 A - 1 4 X - 1 , 34-38c m ) ; 2 = chlor i t e replac ing pyroxene (Sample 8 5 7 D - 3 6 R - 1 ,3-5 cm) ; 3 = a lbi t e replac ing plagioclase (Sample 858G-1 R- 1 , 14-16 c m ) ; 4 = c h lo r i t e (Sa m p l e 8 5 8 G - 1 R- 1 , 1 4 - 1 6c m ) .

mon in the lowermost part of the section. In thin sections we foundalso small segregations of a finely crystalline titanium-bearing min-eral (anatase?) that may be authigenic.Zone I is rather weak ly affected b y hydrotherm al activity. Zone IIis characterized b y an increase in silica due to quartzification, indicatedby high quartz content and by decrease in feldspars, and abundantprecipitation of sulfate. Pyrite, epidote, ankerite (?), and titanium-bearing minerals are detected as traces. Zone III is marked by disap-pearance of feldspars and sulfates, decrease in mica, and appearanceof analcime and wairakite. Chlorite, epidote, rutile, anatase, barite, andpyrite are present in trace am ounts. Quartzification is apparent.Holes 858F and G

Basalt samples from Holes 858F and 858G are strongly altered,mainly to chlorite (Table 4). The chlorite replaces primary mineralsboth in phenocrysts and in the groundmass and infills pores, cracks,and am ygdules. In the basalt sequence at Site 858 the Fe content of thesecondary chlorite decreases upward, from Fe chlorite below (Table 5)to a high-Mg corrensite-like mineral above (i.e., the same trend asdescribed for Ho le 857C ). Albitization of plagioclase was detected inHole 858G (Tables 5). Plagioclase has also been replaced by op al andchalcedony. In the lowermost sample studied, plagioclase is replacedby albite and ep idote, and pyroxene is replaced by chlorite and leu-coxene. Amyg dules are infilled by chlorite, epidote, and quartz. Ground-mass glass is chloritized and epidotized. XRD data show presence ofquartz and amphibole in trace amounts.

CORRELATION AND PROCESSESSites 856,8 57, and 858 can be arranged according to the degree andcharacter of hydrothermal alteration of the sedimentary sequencesrecovered. Site 856 is least affected, especially Hole 85 6A. The com -posite section drilled at Site 857 seems to be intermediate, as the alter-ation zones are expanded over greater thickness and occur at greaterdepths. Site 858 shows the most extensive alteration at shallowestdepths, with "condensed" hydrothermal zones within the shortest in-tervals. However, the mode of hydrothermal processes and the zona-tion seem to be alm ost the same throughou t the area of Midd le Valleydrilled. The sites differ from each other by de gree and intensity of thehydrothermal alteration, but not by the main trends of mineralogicalchanges of the primary terrigeneous sediments. Moreover, the basaltsfrom the sills are altered in the same way as the surrounding andintercalated sedim ents.

Zone IIIZone III is present in all holes except Hole 856A. The zone ischaracterized by strong hydrothermal alteration of sediments and

126


15/19


16/19

V KURNOSOV ET AL.

Z o n e IIZ o n e II was observed in all holes. The main hydrothermal miner-

als for the zone are pyrite, chlorite, and/or a corrensite- like mineral.Anhydrite, quartz, and carbonate are present, and epidote was foundin sediments recovered at Site 857.

Sulfide precipitation as best exhibited by formation of semimas-sive sulfide recovered in Holes 856B and 858B is dominated by pyriteand pyrrhotite. Formation of these minerals requires considerableamounts of free iron to tie up the sulfur. Otherwise the latter migrateswith circulating interstitial water as dissolved H 2 S . When oxygenatedconditions are encountered, H 2S is oxidized to sulfate, which reactswith Ba2+ and C a 2 + ions to form barite and anhydrite. Anhydrite cansubsequently be hydrated and replaced by gypsum at lower tempera-tures. The limiting role of Fe availability was suggested for earlydiagenetic sulfide formation (Volkov, 1984), but the mechanism per-haps works also in the hydrothermal sulfur system.

The complicated interplay between redox processes, iron andsulfur input from hydrothermal fluids, and the thermodynamic poten-tial of H 2O as a factor of hydration- dehydration (Marakushev, 1968)leads to complicated interrelations between the sulfates and sulfidesprecipitated from the fluids into sediment interstices and fissures. Theprocesses are apparently multiple, repeatedly alternating with eachot h e r in the hydrothermal system active today. Experimental datashow that anhydrite forms at 200- 300C (Hajash and Archer, 1980;Seyfried and Bischoff, 1981).

The corrensite- like mineral (mixed- layer smectite/chlorite) we in-terpret as a metastable phase that is a precursor to chloritization. Itforms during active and rapid hydrothermal alteration under relativelyhigh temperature conditions within the active vent area (Hole 858B),where it constitutes almost the total clay mineral content in a zonebetween the smectite-bearing zone above and the chlorite zone below.This assumption is consistent with the thermodynamics of meta-morphic reactions in the system MgO- Al2O3- SiO2 with excess water,which lead to the following progressive alteration parageneses withincreasing temperature: Montmorillonite - Montmorillonite + chlorite(= corrensite?) - Montmorillonite + chlorite + quartz - chlorite +quartz, at temperatures between 350^50C (Marakushev, 1968). Thetemperatures are obtained experimentally for 2 Kbar water pressureand should be diminished under lower fluid pressure conditions. HighMg activity is probably required for abundant corrensite formation.

Carbonates reflect decreasing temperature of hydrothermal fluids.Carbonates of hydrothermal (or of hydrothermally intensified diage-netic) origin are represented mainly by low-magnesium calcite, whichoccurs as concretions (described by Davis, Mott l , Fisher, et al. [1992]but not included in our samples), sparry cement, finely crystallinemicritic matrix, and recrystallization of foraminiferal tests. Abundantcalcite precipitation is confined to the uppermost alteration zone char-acterized by the lowest temperatures and increased alkalinity and pHvalues. The main source of the calcite constituents is likely dissolutionand recrystallization of biogenic carbonates below and within thecalcification zone. However, carbon isotopic analyses (Baker et al.,this volume) show low negative values of 13 C for authigenic carbon-ate from the concretions, indicative of methane- derived C O 2 as asource of carbon. This is consistent with our data obtained from a Kurilisland arc submarine cold spring associated with methane hydrates(Zon en sha i n et al., 1987).Abundant diagenetic calcite/aragonite crustsand concretions were observed there both on the seafloor and withinH o lo c e n e organic-rich volcaniclastic sediments. The 13 C measured inthose authigenic carbonates is as low as - 49% We interpreted thisvalue to be a result of bacterial methane oxidation, the methane beingderived from a rather mature hydrocarbon source deep within thesediment sequence. In the case of Middle Valley carbonate concre-tions the source may be hydrothermally metamorphosed (maturated)organic carbon. Other carbonates found in trace amounts are dolomite,ankerite(?), and aragonite(?).

Z o n e IZ o n e I was found in all holes except Hole 856B. The zone coin-

cides with the lithologic U n i t I. In Hole 856A the zone containsmainly clastic "background" sediment nearly free of smectites. AtSites 857 and 858 the zone contains authigenic smectites.

Smectite is most abundant in H ole 858B, drilled directly into theactive vent area, where the upper unit is represented by metalliferoushemipelagic clay. The hydrothermally derived smectite is confined toth e hemipelagic clay because this clay is accumulating continuouslyby "no rmal" sedimentation at a rate comparable to that of the hydro-thermal input. The underlying turbidites as products of "catastrophic"sedimentation are derived from the nearby continental slope and aredeposited instantaneously, having no time to incorporate a hydrother-mal contribution. An alternative explanation, that smectite disappearsat depths beneath the surface layer by hydrothermal alteration, is lessprobable, as the thermal stability of smectites extends to much highertemperatures than those suggested for the weakly altered upper zone.

SUMMARY AND C O N C L U S I O N SThe mineralogical data on the hydrothermally altered sediments

from Sites 856, 857, and 858 generally confirm the preliminary con-clusions made by shipboard scientists, but reveal addit ional findings.

Sites 856, 857, and 858 can be arranged according to the degreeof hydrothermal alteration of the sedimentary sequences recovered.Site 856 is least affected, especially H ole 856A. The deep compositesection drilled at Site 857 seems to be intermediate, as the alterationzones occur deeper and over greater thickness. Site 858 best showsth e extensive alteration at shallowest depths, with the "condensed"hydrothermal zones within the shortest intervals. However, the modeof hydrothermal processes and the zonation seem to be almost thesame th roughout Middle Valley.

The newly formed hydrothermal minerals found in sedimentsinclude silicates- aluminosilicates (quartz, albitef?], chlorite, cor-rensite- like mineral, smectites, hydromica, epidote group, fibrousamphibolef?], garnetf?], wairakite, analcime, and talc), titanium-bearing minerals (rutile, anatase, sphene, and leucoxene) , sulfates(anhydrite, gypsum, and barite), sulfides (pyrite and others notincluded in our study), and carbonates (calcite, dolomite, and arago-ni te[?]) . The minerals occurring in abundant or common quantitiesare in bold; others are rare. The hydrothermal minerals listed havegrown both in free space (cracks, pores, cavities, molds after dissolu-t ion of anhydrite, foraminiferal tests, empty burrows) and replacingprimary detrital minerals. Both modes of crystallization occur whencrystals (e.g., epidote) formed in interstitial spaces penetrate into thedetrital grains. Recrystallization (regeneration) of quartz leading toquartz cementation is common in silicified sandstones. The primarydetrital chlorite is perhaps also recrystallized during total chloritiza-t ion of the sediments. Skeletal crystals and poikiloblasts of sulfides,anhydrite, gypsum, and sparry calcite occur as cementation of rela-tively fine-grained sediments, including silt and silty clay.

Along with crystallization of new mineral phases and replacementor recrystallization of primary minerals, the hydrothermal altera-t ion includes partial or total destruction of the primary components.Among these processes are dissolution of biogenic carbonate andsilica and destruct ion of detrital feldspars and mica. Dissolution ofanhydrite leading to formation of rectangular molds is another suchdestructive process.

Three main hydrothermal zones are recognized in the sedimentsequence at Sites 856, 857, and 858. The lowermost Zone III withstrong hydrothermal alteration is characterized by pyrite, chlorite,quartz, and epidote, wairakite, and analcime. The mineral assemblagesboth in altered sediments and in associated basalts were formed undergreenschist metamorphic facies. Zone II with moderate hydrothermalalteration contains pyrite, chlorite, corrensite-like mineral, anhydrite,

128


17/19

MINERALOGY OF SEDIMENTS AND IGNEOUS ROCKSquartz, and carbonate. The uppermost Zone I contains mainly clasticsediments and authigenic smectites.

ACKNOWLEDGMENTSWe thank the reviewers for useful remarks. We are grateful to O.Chudaev, E. Petrach enko, V. Vasilyeva, A. Sokolova, E. Pokrovskaya,and V. Sapin for help with analytical work and data processing.

REFERENCES*Cann, J.R., 1969. Spilites from the Carlsberg Ridge, Indian Ocean. J. Petrol,10:1-19.Davis, E.E., Mottl, M.J., Fisher, A.T., et al., 1992. Proc. ODP, Init. Repts., 139:College Station, TX (Ocean Drilling Program).Hajash, A. , and Archer, P., 1980. Experimental seawater/basalt interaction:effects of cooling. Contrib. Mineral. Petrol, 75:1-13.Kholodkevich, I.V., Kotov, N.V., and Kurnosov, V.B., 1981. Experimentalstudy of the alteration of glassy volcanic rocks in distilled water andmodelled seawater at high PT parameters. In Kossovskaya, A.G. (Ed.),Mineralogical Alteration of the Oceanic Rocks: Moscow (Science), 67 -72 .Kurnosov, V.B., 1986. Hydrothermal Alteration of Basalts in the PacificOcean and Metal-bearing Deposits, Using Data of Deep-sea Drilling:Moscow (Izd. Nauka).Marakushev, A.A., 1968. Thermodynamics of the Metamorphic Hydration ofMinerals: Moscow (Nauka).

Melson , W.G., and Van Ande l, T.H., 1966. Metamorphism in the Mid-Atlanticridge 22N latitude. Mar. Geol, 4:234-246.Murdmaa, I.O., and P rokoptsev, N.G., 1968. On the recovery of spilite in theArabian-Indian Ridge rift zone. Dokl Acad. Nauk, 181:458-461.Rozanova, T.V., Driz, V.A., and Dmitrik, A.V., 1978. A hydrothermal pyrox-ene-amphibole asbestos rock from the Hess Deep. Litol. Polezn. Iskop.,3:3-16.Seyfried, W.E., and Bischoff, J.L., 1981. Experimental seawater-basalt inter-action at 300C, 500 bars, chemical exchange, secondary m ineral forma-tion and implications for the transport of heavy-metals. Geochim.Cosmochim. Ada, 45:135.Sudo, T, Oinuma, K., and Kobayashi, K., 1969. Mineralogical problemsconcerning rapid clay mineral analysis of sedimentary rocks. Acta Univ.Carol, Geol Suppl., 1:189-219.Volkov, I.I., 1984. Geochemistry of Sulfur in the Ocean: Moscow (Nauka).Zonenshain, L.P., Murdmaa, I.O., and B aranov, B.V., 1987. Submarine gas-spring in the Sea of Okhotsk West from the Paramushir Island. Okeanolo-gia (Moscow), 27:795-800.

Abbreviations for names of organizations and pu blications in ODP reference lists followthe style given in Chemical Abstracts Service Source Index (published by AmericanChemical Society).Date of initial receipt: 23 February 1993Date of acceptance: 16 September 1993Ms 139SR-267

129


18/19

V . K U R N O S O V E T A L .

Plate 1. SEM micrographs of hydrothermal minerals from Leg 139 sediments: clay minerals. 1, 2. Radiated hydromica aggregate: Sample 139-856B-15X-4,73-7 5 cm, 2730. (1) micrograph. (2) scanning microprobe measure of K. 3. Elongated platy crystals of hydromica in radiated aggregate: Sample 139-856A-13X-4,94-98 cm , 2730. 4-7 . Different forms of chlorite: (4) rosette-like aggregate of curved flakes; Sam ple 139-856B-15X-3,112-114cm,2730. (5) radiatedaggregate; Sample 139-858A-39X-CC, 15-17 cm, 1820. (6) platy crystals; Sample 139-858A-21X-3, 19-22 cm , 4550. (7) cabbage-like spheroidal aggregates;Sample 139-858A-39X-CC, 15-17 cm, 1820. 8. Corrensite-like (mixed-layer) mineral; Sample 139-858B-5H-4, 14-18 cm, 1820.

130


19/19

MINERALOGY OF SEDIMENTS AND IGNEOUS ROCKS

I

Plate 2. SEM micrographs of hydrothermal minerals from Leg 139 sediments: nonclay minerals. 1. Fibrous-diverse aggregate of amphibole (?) infillingrecrystallized foraminiferal test; Sample 139-856B-2H-3, 106-110 cm, 4550. 2-4 . Different habits of gypsum crystals. (2) Sample 139-858D-4H-4, 100-10 4cm, 1820. (3) Sample 139-858D-4H-4, 100-104 cm, 910. (4) Sample 139-858C-6H-3, 34-36 cm, 2730. 5-6. Euhedral epidote crystals growing in porespace: (5) Sample 139-857D-31R-1, 13-15 cm, 4550. (6) Sample 139-857D-37R-1, 42-43 cm, 4550. IS. Wairakite and analcime crystals in quartz-wairakite-analcime vein in chloritized claystone matrix; Sample 139-858A-39X-CC, 15-17 cm, 3640.

Documents

Mineralogia Rocks